1. Field of the Invention
The present invention relates to a tyre, for a motor vehicle, with a low rolling resistance (R.R.). More particularly, the present invention relates to a tyre having a combination of sidewall rubber mixture/carcass ply coating rubber mixture able to reduce R.R. of the tyre.
2. Description of the Related Art
One of the most pressing requirements of motor vehicle manufacturers, with regard to the tyre performance, is a low R.R.
In view of this requirement, tyre manufacturers have hitherto made great efforts to reduce R.R. without adversely affecting other important characteristics such as handling, comfort and wear resistance.
Among the various components which make up a tyre, the component which mostly influences the R.R. is clearly the tread band since it is the component which comes into direct contact with the ground.
Therefore, during recent years, technicians in the field have concentrated their efforts on modifications of the tread rubber mixture. In particular, attempts have been made to modify hysteretic characteristics of the tread rubber mixtures by modifying, in particular, the loss factor defined as tan δ=E″/E′, where E′=elastic modulus (storage modulus), E″=viscous modulus (loss modulus). Technicians in the field consider indeed that an excellent compromise between a low R.R. and a good wet grip of the tyre can be obtained by means of a rubber mixture which has a low value of tan δ at medium temperatures (50–70° C.) and a high value of tan δ at low temperatures (0–10° C.).
In this respect, a particularly remarkable modification in the hysteretic behaviour of the rubber mixtures was obtained by replacing, totally or partly, conventional reinforcing fillers based on carbon black with the so-called “white fillers” and in particular with silica (see, for example, patent EP-501,227).
In addition, attempts have been made to improve the R.R. by modifying the rubber mixture of the tread underlayer and of the carcass plies or of the sidewalls of the tyre.
U.S. Pat. No. 4,319,619 describes a radial tyre in which the rubber portion of at least one carcass ply and the tread underlayer are made from a rubber having a viscoelastic property tan δ≦0.2 and an elastic modulus≧120 Kg/cm2. In that tyre the R.R. would be reduced without worsening behaviour during braking, stability, comfort and wear resistance.
According to U.S. Pat. No. 5,929,157 the R.R. of a tyre may be reduced, without reducing wear resistance and handling on wet ground and without increasing electric resistance of the tyre as a whole, when the rubber mixture of the tyre sidewall is produced using, as a reinforcing filler, a particular type of carbon black, partly replacing carbon black with a particular type of silica and, in particular, using a specific quantity of a silane as coupling agent (col. 1, lines 32–39).
It is a common belief that hysteretic characteristics of other parts of the tyre do not have a remarkable influence on the overall R.R. of the tyre. This fact is confirmed by calculations on models performed using finite-element analysis [see, for example, J. L. Locatelli and Y. De Puydt in “Tire Technology International” (June 1999, pages 50–55)].
Modification of other parts of the tyre in order to reduce the R.R. is also not recommended since, in an attempt of achieving a possible slight improvement in the R.R., a risk is run of negatively influencing other important characteristics and, therefore, of worsening the overall tyre performance.